1. Field of the Invention
The present invention relates to low free formaldehyde dispersions of melamine crystals in alkaline resole resins and methods of making such dispersions.
2. Background Discussion
Melamine-resole compositions are high water solubility compositions finding use for bonding materials. These compositions are especially useful for bonding glass fiber to make an initially "resinated" glass fiber blanket or mat that is subsequent thermally cured in a mold or some other shaping operation.
High water solubility is normally provided by formulating these resins (e.g., phenol/formaldehyde-resole resins or melamine/formaldehyde resins) with high formaldehyde to phenol molar ratios or with high formaldehyde to melamine molar ratios (near or above 3:1 in either case). Reaction products are respectively monomeric methylol phenols and methylol melamines. The methylol melamines are normally stabilized by further reaction with methanol to form the water soluble methoxy methyl melamine derivatives. A high content of unreacted formaldehyde is therefore often found in prior art resins of these types as a consequence of the high formaldehyde ratios employed to prepare them.
Typically, mixtures of resoles and melamines are heated to effect a melamine formaldehyde reaction to produce a dissolved methylol melamine reaction product (See U.S. Pat. No. 4,960,826). Application of heat to thermally set (polymerize) these types of conventional resole resins in curing operations also causes the release of additional amounts of formaldehyde from condensing methylol groups in the resole resins and from condensing methoxy methyl groups in the melamine resins. The release of irritating formaldehyde gas from resin impregnated, i.e., resinated, glass wool or glass fabrics during molding and shaping operations may be objectionable from an environmental or industrial hygienic point of view.
Formaldehyde release during curing of the resin is also a problem with the well-documented use of mixtures of these two types of resins. It has therefore been necessary and common industrial practice to control formaldehyde evolution into the environment and work place during curing and B-stage (when resin is mixed with substrate) operations, by compounding these methylol phenol and methoxy methyl melamine binders, or mixtures of the two, with significant amounts of urea and/or ammonia. Urea and ammonia have very high reactivities towards free or uncombined formaldehyde and readily affect its capture from aqueous A-stage (prior to mixture with substrate) solution compositions. The products from such scavenging reactions are dissolved methylol urea monomers and hexamine (also called hexamethylenetetramine), respectively. Mixtures containing methylol ureas may still be problematic, however, because the thermally-induced condensation reaction between methylol ureas also releases formaldehyde.
Additionally, the use of urea and particularly ammonia as formaldehyde scavengers results in the formation and release of undesirable trimethylamine through various, thermally-induced decomposition reactions. The formation of trimethyl amine from ammonia and formaldehyde is a well-documented reaction. Trimethylamine, although possibly not as volatile or irritating as formaldehyde, does have an objectionable, foul smelling "fishy" odor. Trimethylamine also has a low human threshold odor limit in the low parts per billion range.
The trimethylamine is particularly noticeable in high density molded articles prepared with high binder contents because the curing operation traps evolved gasses inside the mold, and subsequently inside the glass fiber resin matrix. These gasses give the finished article a long-lasting unpleasant trimethylamine odor. Less dense articles, such as residential thermal insulation that are cured by blowing heated air through the resinated glass wool blanket, may have less odor in the finished article. The escaping trimethylamine gas purged from the curing blanket by the hot air flow nonetheless ends up in the environment. The use of urea and/or ammonia to scavenge or capture formaldehyde in these binders therefore may not satisfy environmental restrictions on fiberglass plants and the increasing quality (odor free) standards for finished products.
These decomposition reactions occur as a result of the inherent thermal instability of urea and its reaction products with formaldehyde and the very high curing temperatures employed; (often above 500.degree. F.) to mold or permanently shape resinated glass wool.